The combination of high toxicity and widespread occurrence has created a pressing need for effective monitoring of arsenic in groundwater. Through this program Agiltron will develop a field deployable monitoring tool based on surface enhanced Raman spectroscopy (SERS) for rapid on-site detection and quantification of arsenic in groundwater. During operation in the field, the SERS sensor module can be inserted directly into samples of groundwater brought up to the surface or lowered directly into the water well with the use of long fiber length for on-site arsenic detection and quantification. The proposed system will be able to detect arsenic in groundwater at 1 ppb concentration level, in situ, on-site, with little or no sample preparation, and with no interferences. During Phase I we have successfully demonstrated the detection of 1 ppb level arsenic in water samples using the proposed portable SERS system. In Phase II the performance of the proposed SERS system will be optimized for highest throughput at the wavelength of interest by using suitable excitation wavelength and SERS substrates with optimized localized surface plasmon resonance properties. On-site measurement of arsenic in groundwater in local water wells will be conducted. The feedback from these field studies will be used to further develop/improve the performance of individual components so that an optimized monitoring system suitable for practical site operation can be developed and are ready for manufacture.